One expected use case for this (and the reason why it
was first created) is to work around problems with an
index directory accessed via filesystems like NFS because
NFS does not provide the "delete on last close" semantics
that Lucene's "point in time" search normally relies on.
By implementing a custom deletion policy, such as "a
commit is only removed once it has been stale for more
than X minutes", you can give your readers time to
refresh to the new commit before IndexWriter
removes the old commits. Note that doing so will
increase the storage requirements of the index. See LUCENE-710
for details.

onInit

This is called once when a writer is first
instantiated to give the policy a chance to remove old
commit points.

The writer locates all index commits present in the
index directory and calls this method. The policy may
choose to delete some of the commit points, doing so by
calling method delete()
of IndexCommit.

Note: the last CommitPoint is the most recent one,
i.e. the "front index state". Be careful not to delete it,
unless you know for sure what you are doing, and unless
you can afford to lose the index content while doing that.

Note: the last CommitPoint is the most recent one,
i.e. the "front index state". Be careful not to delete it,
unless you know for sure what you are doing, and unless
you can afford to lose the index content while doing that.

Parameters:

commits - List of IndexCommit,
sorted by age (the 0th one is the oldest commit).